1. Field of Invention
The present invention relates to a load-supporting apparatus and, more particularly, to a linkage for supporting a load.
2. Related Prior Art
As disclosed in U.S. Pat. No. 6,478,275, a conventional support device includes a base 10, a bracket 20, an arm 30, a lever 40 and a spring 50. The bracket 20 is used to carry an object. The arm 30 is pivotally secured to the base 10 with a shaft 31. The lever 40 is secured to the base 10 with a rod 42. The lever 40 includes a curved groove 43 for slidably receiving the shaft 31 and an orifice 41 for loosely receiving the rod 42. The base 10 includes a stop 11 for engaging and latching the lever 40 with the base 10, without additional fasteners, when the lever 40 is elevated relative to the base 10. The spring 50 includes an end placed against the rod 42 and another end placed against the arm 30 so that the spring 50 tends to keep the arm 30 in a vertical position when the base 10 is placed on a horizontal surface. The use of this conventional support device is however not without any problem. At first, the support device is not adaptable to the height of a user. A tall user would like to have the support device to pivot in a small range so that the position of the object such as a display would not be too low. A short user would like to have the support device to pivot in a large range so that the position of the object could be adequately low. Such adjustment is impossible without any device for adjusting the preload of the spring 50. Moreover, the support device is allowed to pivot in a range limited by slots 33 made in the arm 30 and slots 43 made in the lever 40.
As disclosed in US Patent Application Publication No. 2011/0260017, a conventional adjustable support arm includes a parallelogram linkage that includes a lower bracket 210, an upper bracket 290, an upper link 240 and a lower link 250. A spring 260 counterbalances the weight of an object carried on the upper bracket 290. A first end of the spring 260 is attached to the upper link 240 by a threaded bolt 268 and a screw cap 269. The threaded bolt 268 and the screw cap 269 are used to adjust preload of the spring 260. A second end of the spring 260 is attached to a drag link 280 to provide a dynamic attachment point that automatically adjusts as the support arm is moved between upper and lower positions. Two pivot links 270 and 275 or two slots 253 and 255 in the lower link 250 are used to guide the movement of the drag link 280 as the height of the support arm is adjusted. The support arm is adaptable to a tall user by rotating the threaded bolt 268 relative to the screw cap 269 in a direction to increase the preload of the spring 260. The support arm is adaptable to a short user by rotating the threaded bolt 268 relative to the screw cap 269 in an opposite direction to reduce the preload of the spring 260. However, the support arm is allowed to pivot in a range limited by the length of the pivot links 270 and 275 or the slots 253 and 255.
Therefore, the present invention is intended to obviate or at least alleviate the problems encountered in the prior art.